Polycarbonate resins have been known to have the highest impact resistance of the engineering plastics and to have good heat resistance, and have been used in various fields by utilizing these characteristics. However, the polycarbonate resins have drawbacks such as poor chemical resistance, poor moldability, and dependence of impact strength on their thickness.
Thermoplastic polyesters have excellent chemical resistance and moldability but have drawbacks such as poor impact resistance and poor dimensional stability.
In order to take advantage of the characteristics of each material and to compensate for their drawbacks, various resin compositions including a combination of the polycarbonate resin and the thermoplastic polyester resin have been developed and studied to simultaneously satisfy characteristics required for automobile parts and other products, such as impact resistance, heat resistance, chemical resistance, weatherability, and moldability.
A resin composition containing the polycarbonate resin and a polyester modified with polyethylene glycol, polytetramethylene glycol, or the like has also been developed. Such a resin composition has higher moldability but may have insufficient heat resistance for exterior parts of automobiles.
Separately, a resin composition containing the polycarbonate resin and a polyester-polyether copolymer comprising a polyalkylene glycol adduct of a bisphenol as a block unit and being prepared by using a germanium catalyst has been developed. For example, Patent Document 1 discloses a resin composition containing 30 parts of a polyethylene terephthalate block copolymer composed of 30% of bisphenol A polyethylene oxide adduct having a molecular weight of 1,000 and 70 parts of polycarbonate. The resin composition has an excellent balance of moldability, heat resistance, and impact resistance without impairing a surface appearance of a moulded body of the composition. Here, the disclosure suggests that the addition of a graft copolymer containing an elastomer as an impact modifier enables an improvement of the impact resistance.
At present, to housings of electrical appliances such as cell phones and personal computers and vehicle parts such as spoilers, fenders, door panels, and back door panels for automobiles, there are strong requirement to further reduce the weights, and thus moulded bodies are required to be thinner. When the thickness of a moulded body is actually tried to be reduced, the resin composition of Patent Document 1, which is excellent as a molding material, is unfortunately, likely to become short shot during injection molding in some cases. In order to solve the short shot, when the number of gates is increased, the number of welds are increased, and this may reduce the strength of a moulded body. In order to solve the short shot, when the moulding temperature is set higher for improving the flowability, a resin composition is consequently, thermally degraded to generate a pyrolysis gas, leading to a poor appearance, in some cases. When produced by using a resin composition similar to a conventional one and by using a mold designed with the same idea as the conventional one, the moulded body having a smaller thickness may have a lower rigidity as a whole, and such a new problem is required to be improved.
In order to give rigidity to a moulded body having a small thickness as a whole and to reinforce the moulded body, for example, a planer structure, which is called a rib, standing perpendicular to a back surface, can be provided on the back surface of the main body of the moulded body opposite to a design surface. This method unfortunately has a problem. In other words, a rib part having an excessively large thickness exerts high rigidity improvement effect but generates a hollow called a sink mark on the design surface of the outer surface of a molded product along the rib part, thus deteriorating the appearance. In order to prevent the sink mark to improve the appearance, a method of reducing the thickness of the rib part is commonly employed. In some cases by using such a method, the rib part cannot be filled with a resin composition, for example, and thus the method cannot achieve the balance between sufficient reinforcement effect and sink mark prevention. Moreover, a moulded body having a smaller thickness has a lower rigidity and thus required more effective reinforcement. Such a moulded body itself cannot be sufficiently filled with a resin composition in addition to the rib part, resulting in an unsatisfactory moulded body, in some cases. Even when the moulded body is obtained, the rib part is insufficiently filled with a resin composition, failing to achieve the reinforcement effect, for example. Conventional technology has a limitation to achieve a smaller thickness.
In order to sufficiently fill the rib part to improve the rigidity and to prevent the sink mark, examples in Patent Document 2 describe a reduced thickness structure where the rib has a thinner base. Patent Document 3 disclosed an automobile side molding having positioning bosses on its back surface. As with the case having ribs, Patent Document 3 disclosed a technique of reducing the thickness of the base of the boss, or a reduced thickness structure around the base of the boss, in order to prevent sink marks from generating on a design surface opposite to the boss parts and to sufficiently fill the bosses themselves with a resin composition to work the bosses. Patent Document 4 discloses a molded product having a rib part on its back surface, and the thickness of the main body of the molded product is reduced from the back surface at both sides of the rib part, that is, reduced thickness parts are formed on the back surface of the main body.